Sonnet Morpheus D/A processor Measurements

Sidebar 3: Measurements

I measured the Sonnet Morpheus with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), repeating some tests with the magazine's Audio Precision APx555 system. The AES3 and coaxial S/PDIF inputs accepted 16- and 24-bit data sampled at rates up to 192kHz; the optical TosLink input accepted data sampled up to 96kHz. I wasn't able to examine the Morpheus's performance via its USB input because this was fitted with a RJ45 jack, presumably for an I2S connection from Sonnet's Hermes network bridge, which I didn't have at hand for the measurements.

Although the Morpheus has a volume control, I set the processor to Fixed Volume for the measurements, which is how I understood AH had auditioned it. The Morpheus's maximum output level at 1kHz was 4.21V from the balanced outputs, 2.12V from the single-ended outputs. Both outputs preserved absolute polarity (ie, were noninverting). The single-ended output impedance was a low 99 ohms at all audio frequencies, the balanced output impedance 98 ohms at 20Hz and 1kHz, dropping slightly to 92 ohms at 20kHz.

The Sonnet is a nonoversampling design that lacks a reconstruction filter. The impulse response is therefore transient-perfect (fig.1; ignore the small amount of pre- and post-ringing in this graph, which is due to the Audio Precision's anti-aliasing filter, operating at a sample rate of 200kHz). The trade-off for this optimized time-domain behavior is a discontinuous output waveform (fig.2) and an extremely slow ultrasonic rolloff, with nulls at the sample frequency and its harmonics (fig.3, magenta and red traces). The lack of a reconstruction filter also means that there is very little attenuation of aliased images (footnote 1). The aliased image at 25kHz of a full-scale tone at 19.1kHz (cyan, blue) is hardly suppressed at all, and many other images are visible in this graph. The harmonics associated with the 19.1kHz tone are all very low in level, however.

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Fig.1 Sonnet Morpheus, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).

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Fig.2 Sonnet Morpheus, waveform of 1kHz tone (2ms time window).

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Fig.3 Sonnet Morpheus, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) into 100k ohms with data sampled at 44.1kHz (20dB/vertical div.).

Fig.4 shows the frequency response with data sampled at 44.1, 96, and 192kHz. The output is down by 1dB at the top of the audioband with 44.1kHz data (blue and green traces). There is a smooth ultrasonic rolloff with 96kHz data (cyan, magenta traces) and 192kHz data (gray, red traces). Channel separation was superb, at >125dB in both directions below 1kHz (fig.5), decreasing to a still-superb 113dB at 20kHz. The Morpheus's noisefloor was free from any power supply–related spuriae (fig.6).

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Fig.4 Sonnet Morpheus, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right blue), 96kHz (left cyan, right magenta), and 192kHz (left gray, right red) (1dB/vertical div.).

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Fig.5 Sonnet Morpheus, channel separation (5dB/vertical div.).

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Fig.6 Sonnet Morpheus, spectrum with noise and spuriae of dithered 1kHz tone at 0dBFS with 24-bit data (left channel blue, right red) (20dB/vertical div.).

Linearity error with 24-bit data (fig.7) was negligible down to –120dBFS, which implies high resolution. An increase in bit depth from 16 to 24, with dithered data representing a 1kHz tone at –90dBFS, dropped the Sonnet's noisefloor by around 24dB (fig.8). This implies a resolution of 20 bits, which is excellent. When I played undithered data representing a tone at exactly –90.31dBFS, the waveform was symmetrical, with negligible DC offset, and the three DC voltage levels described by the data were free from noise (fig.9). With undithered 24-bit data (fig.10), the Morpheus's very low analog noisefloor means it can output a clean sinewave, even at this very low signal level.

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Fig.7 Sonnet Morpheus, left channel, 1kHz output level vs data level in dBFS (blue, 20dB/vertical div., left); linearity error (red, 1dB/vertical div., right).

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Fig.8 Sonnet Morpheus, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).

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Fig.9 Sonnet Morpheus, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

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Fig.10 Sonnet Morpheus, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).

The Morpheus produced very low levels of harmonic distortion with full-scale data (fig.11). The third and fifth harmonics are the highest in level, at just –110dB (0.0003%), and though other harmonics are visible in this graph, these are all 10–20dB lower in level. This spectrum was taken into the high 100k ohms load. The levels of the harmonics didn't rise when I reduced the load impedance to the stressful 600 ohms. Intermodulation distortion with an equal mix of 19 and 20kHz tones, each lying at –6dBFS, was low, with the difference tone at 1kHz lying at –84dB (0.006%, fig.12). However, the test tones generated many aliased products, both in the audioband and above half the sample rate. Reducing the signal level by 3dB didn't alter the picture (not shown).

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Fig.11 Sonnet Morpheus, 24-bit data, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

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Fig.12 Sonnet Morpheus, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms, 24-bit, 44.1kHz data (left channel blue, right red; linear frequency scale).

Fig.13 shows the spectrum of the Morpheus's output when its TosLink was fed high-level, 16-bit J-Test data. All the odd-order harmonics of the undithered low-frequency, LSB-level squarewave lie well above the correct levels, these indicated by the sloping green line. A pair of sidebands of unknown origin is also present at ±817Hz. I repeated this test with 24-bit J-Test data and with AES3 data but got the same result.

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Fig.13 Sonnet Morpheus, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit TosLink data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Some of the Sonnet Morpheus's measured performance—the optimal time-domain behavior, against which must be set the production of high-level aliased images—is the inevitable result of the decision not to use a reconstruction filter. The Morpheus does offer excellent resolution, a very low noisefloor, and superb channel separation, but its disappointing rejection of word-clock jitter via its AES3 and S/PDIF inputs suggests something is not right with the receiver used for these inputs. It is possible, however, that the USB and I2S inputs, which bypass the receiver, don't suffer from this problem.—John Atkinson


Footnote 1: See my discussion of why a reconstruction filter is necessary here.
COMPANY INFO
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COMMENTS
windansea's picture

I keep on hearing about musicality of R2R DACs, but of the various components in the signal chain, I say the DAC ranks with cables as least-detectable element, with speakers then amp as most detectable. I sure wish Stereophile would include a little bit of ABX DBT analysis to tease out the significance of any differences. For example, if a delta-sigma DAC has oversampling turned off, does it sound indistinguishable from an R2R DAC? Is it actually the oversampling that lessens "musicality"?

That said, I've yet to listen to an R2R DAC yet, so I'll confess my ignorance.

Axiom05's picture

A Delta-Sigma DAC requires oversampling since the DAC is only working with a "few" bits at a time. For me, the main interest of a NOS DAC is so one can use something like HQPlayer and explore oversampling and digital filtering in the software. You can use a powerful computer to do this part and with a NOS DAC you don't have an additional digital filter to "stack" on top of.

Archimago's picture

While an SDM DAC will still convert the PCM input to typically multi-bit SDM, that doesn't mean it can't turn off the oversampling applied.

You can still get a "stair-stepped" NOS 44.1kHz playback (for example) with various DACs like the RME ADI-2 series or even the old TEAC UD-501.

Demonstrated here:
TEAC UD-501 - TI/BB PCM1795 "Advanced Segment" DAC:
http://archimago.blogspot.com/2021/10/revisiting-teac-ud-501-dac-2013-thdn.html

RME ADI-2 Pro FS R Black Edition - AKM AK4493 DAC:
http://archimago.blogspot.com/2020/09/measurements-rme-adi-2-pro-fs-r-black_26.html

georgehifi's picture

Seeing this has an output stage (100ohms and over 2v) that can drive any poweramp, it's a real shame it was not used direct using the the two preset switches to match for the loudest it can do into an amp. As this way to me it would be even better, as "the best preamp is no preamp"

Cheers George

Jack L's picture

Hi

Bingo !

Why add more harmonic & phase distortion of any premp to the music signals when a DAC with low impedance & 2V+ O/P can drive direct any power amps ???????

That's exactly what I've been doing with my 24bit/192KHz DAC - hookup to my design/built linestage with PASSIVE bypass switch ON, for a couple years now !!

YES, "the best preamp is NO preamp" !!!!!!!!

So my CD/DVD & streaming music go DIRECT to my all-triode SET via my DAC + passive linestage. The music sounds soooo transparent, fast, detailed with livelike performance environment. No problem of lacking power at all.

I never want to switch the linestage back to active mode again. It simply loses the sonic beauty & powerfullness of the above passive mode.

Listening is believing

Jack L

PS: That said above, I am still an vinyl addict !

Jack L's picture

Hi

"Great minds think alike" quoted d Belchien 1618.

Fully agreed to yr above statement. Why let harmonic & phase distortions add to the music signals passing through an active linestage preamp ???

Be a smart audio consumers by listening to the music performance in stead of the audio store salespersons' sales pitch.

I heard enough 'myths' rumouring any power amps can't deliver full powers without a preamp. Technically it is bullshit. Any CD/DVD players/DAC do deliver more than enough output voltage (2Vrms++) to drive any power amps (commercial made & home-brews like mine) to full output power. Period.

I've been using true passive line-amp since day one many many years back & then phono-preamp with its active linstage switchable to passive bypass. All are my DIYed design/built.

In passive mode, the music improvement over active mode of the same amp is sooo obvious: wide OPEN, airy, crystalline transparent, detail & FAST transient !! FYI, it is the fast transient response (shortest time the music signals swing from zero to their peak levels) gives our ears/brain the perception of the powerfulness (strength & latten energy) of the music performance that active mode come short !!

Playing vinyl does make such sonic difference even more pronounced. I know as I've compared the music by switching back & forth the passive bypass switch so often.

Listening is believing

Jack L

Panos.G's picture

Hi Alex,

Great article and insight on the Sonnet DAC.

I see you used a 3rd party ethernet (RJ45) I2S cable (Phasure ET^2) to connect the Denafrips Iris DDC to the Morpheus. My question here is, do you think that would also work for Singxer SU-6 or for Denafrips Hermes ?
Cause as it seems the I2S connection is the way to go, to get the full benefit of this DAC.

Many thanks
Panos

Nez7's picture

Hi, I am thinking about buying the Sonnet Morpheus as well s getting a Denafrips Hermes DDC to output the i2s signal. Can anyone point me to the specific i2s cable that is compatible with these components?

My understanding is the Sonnet uses an unconventional i2s input so an off the shelve generic i2s cable won't work.

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